Add a Mountain Lion OS X like theme for ttrss.

Now that themes are better-supported via preferences, I thought I'd kick one off. I've called it _Mountain Lion_ because it's inspired by a Reader Plus (for Chrome) theme of the same name.

It's a pleasant-looking theme that looks good on most monitor sizes and works just as well on Mac OS as other OS.

It's intended for "Combined Theme Display" but improves the overall look and feel even when using the split display. Further improvements are welcome.